Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geologic formation, referred to as a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. When drilling the well, a viscous drilling fluid (or drill mud) with certain increased shear strength characteristics is often used to maximize removal of drill cuttings to be transported out of the well. Still, settlement of drill cutting particles must be expected. Another purpose of the drill mud is also to provide a filtercake along the wellbore to prevent fluid loss into the formation. To save rig time, a screen completion is often installed in this drilling mud. This drilling mud containing particles from the drilling phase may to some extent be removed by circulating fresh or conditioned mud into the well or be displaced by a clean completion fluid such as brine. In any case, the well bore may be covered by a filtercake. Once the well is completed, the filtercake may be removed to produce the well. To remove this filtercake, a certain differential pressure is required across the filtercake (i.e., the formation pressure on the formation side of the filtercake must be higher than the pressure on the borehole side of the filtercake to break the filtercake off of the borehole wall).
When the well is to be put on production, the drilling fluid system (including the filtercake) must flow back through the sand screens. If the screen completion becomes long (for example, in the range beyond 300 m to 500 m), it may be a challenge to remove the fluid system properly along the whole interval. Also, a certain pressure drop is required to initiate the flow of reservoir fluid. This is both (1) because of the required differential pressure to lift off the filter cake and (2) to break circulation as the drilling fluid starts flowing in various tortuous paths of the screen completion. In addition, pressure increases along the tubing in long horizontal wells makes it more difficult to achieve the needed liftoff pressure because as you move upstream in the tubing, the pressure increases. Thus the differential pressure between the formation and the tubing decreases as you move from the heel of the well to the toe of the well (also called the heel-toe effect). As explained herein, this decrease in differential pressure will contribute to the difficulty of removing the filtercake along the upstream portions of the wellbore. As the first part (typically high-permeable zones) of the well starts flowing, the required pressure may no longer be available in the remaining part of the well. Consequently, the cleanup process will in many cases stop before the whole well is cleaned up and can contribute.
Accordingly, the flow performance of long, screen-based completions may be a challenge due to problems related to well cleanup.